an opening at the top through which the water flows in such a manner that the head
(vertical height of the water surface above the opening) can be measured. The
openings in a weir may be of many shapes--rectangular, parabolic, trapezoidal, v-notch,
and various combinations of these shapes. One of the easiest to construct and most
versatiIe weirs is the 900 v-notch weir. This type of weir yields accurate measurements
of flow in the range from about 25 to 2,000 gallons per minute. For flows less than 50
gallons per minute, the 60 o v-notch weir is more accurate. For flows greater than 500
gallons per minute, the rectangular weir is more accurate. Figure 5-3 illustrates the
principle of the 90o v-notch weir. The notch should be beveled--preferably with a metal
strip--toward the direction of the flow, so that the water crests over a sharp edge. The
notch is cut at a right angle (90o) with the vertical axis bisecting the angle as shown in
the figure. For optimum accuracy, the following dimensions are critical:
Figure 5-3. Critical dimensions for 90 v-notch weirs.
MD0160
5-5